1. Field of the Invention
The present invention relates to the treatment of comminuted cellulosic material, for example, wood chips, to enhance the properties of the pulp produced from the cellulosic material. More particularly, the present invention relates to the pretreatment of cellulosic material with acidic solutions and carbonate solutions prior to treatment with pulping chemicals.
2. Description of Related Art
Prehydrolysis of wood chips initiated with hot water or dilute acetic acid is an established practice in the pulp and paper industry. For example, U.S. Pat. No. 4,436,586 of Elmore; U.S. Pat. No. 4,668,340 of Sherman; U.S. Pat. No. 4,612,286 of Sherman, et al.; and U.S. Pat. No. 5,589,033 of Tikka, among others, disclose processes for acid hydrolysis pretreatment of wood chips. U.S. Pat. No. 4,652,341 of Prior discloses a process of nitric acid pretreatment of wood chips, and then caustic extraction. However, there is renewed interest in this treatment because the sugars dissolved the acidic solution, that is, the hydrolyzates, can be a source of lignocellulosic ethanol.
Approximately 70% of the hemicelluloses dissolve in the cooking liquor during kraft or soda/anthraquinone (SAQ) pulping. These dissolved hemicelluloses consume a significant fraction of the active cooking chemical, that is, the alkali, in the process of being degraded to low molecular weight compounds. With the present state of pulping technology, it would be technically difficult to remove these low molecular weight compounds from alkaline pulping effluents and purify these compounds into product streams to a degree adequate enough for use as commodity chemicals. The low molecular weight products that are generated from hemicellulose degradation during alkaline pulping have a high oxygen to carbon ratio and relatively low calorific values during combustion.
One team of researchers, Bolton, et al. “Chemical and physical changes due to acidolysis of chips ahead of alkaline pulping.” Proceedings of the 14th International Symposium of Wood, Pulp, Fiber Chemistry, Durban, South Africa, Jun. 25th-28th (herein “Bolton (2007A),” included by reference herein in its entirety), including the present inventor, found that an acid treatment of chips followed by a soda/anthraquinone (SAQ) pulping process provides an improved pulp yield for hardwood chips compared to kraft treatment alone.
From this and other research, the present inventor surmises that reducing end groups (REGs) may be generated when glycosidic bonds are cleaved in an acid (or A-stage) treatment of wood chips. For example, typically when the free A-stage effluent is drained off after acid treatment, the chips retain about 1.0 liters of solution per kilogram of chips (on an oven-dried basis). Typically, this entrained liquid is transferred with the chips to the subsequent alkaline carbonate neutralization stage, if provided, or transferred directly to the chemical pulping stage, for example, to a kraft or a SAQ pulping stage. The inventor further surmises that the reducing end groups in the acid liquor that is transferred to the subsequent treatment both accelerate the rate of SAQ delignification and improves pulp yield. As is known in the art, pulp “yield” of a pulping process is the percent by weight of the delignified pulp fibers (about 90% lignin removal from wood) compared to the weight of the wood chips introduced to the process. A higher pulp yield is preferred.
Specifically, the inventor surmises that the higher concentration of reducing end groups (REG) can reduce anthraquinone (AQ) in the SAQ process at a higher rate to form anthrahydroquinone (AHQ), that is, the active delignification catalyst in the SAQ process. The AQ/AHQ catalytic cycle is shown schematically in FIG. 1. In addition, when a reducing end group in the solid phase is oxidized by AQ to a carboxylic acid it becomes resistant to the alkaline peeling reaction that lowers the molecular weight of carbohydrates resulting in solubilzation and a decrease in pulp yield. The hypothesis above may explain why the A-stage improves pulp yield for SAQ pulping, but not for kraft pulping. For example, the oxidation of reducing end groups to carboxylic acids is not known to be a significant reaction (or even occur) in the kraft process.
However, the present inventor has found it difficult to verify this hypothesis. This difficulty is due to the lack of analytical approaches to test this hypothesis on the significance of carryover of reducing end groups (REG). In Bolton (2007A), the investigators reported that mild acidolysis increased the content of reducing end groups in sugar maple from 0.33 mmole/g to 0.38 mmole/g. In that investigation, the test method was the reduction of dinitrosalicylic acid (DNS) with the reaction products being quantified by visible spectroscopy. The researchers also reported on the effect of purchased birch xylan added directly to SAQ cooking. It is believed that the xylan hydrolyzes during the alkaline pulping stage and generates additional REG. A lower kappa number (that is, an indication of the presence of undesirable lignin in the cellulose pulp, where wt. % of lignin≈0.15×kappa number) was obtained as well as less rejects. However, additional research by Bolton, et al. 2007. “A biorefinery approach: Nonsulfur pulping with partial lignin recovery and conversion.” 2007 TAPPI Environ. Pulping and Engineering Conference, Jacksonville, Fla. (herein “Bolton (2007B),” included by reference herein in its entirety), including the present inventor, included no mention of any quantification of REG because of suspicion that the DNS method is unreliable when the sample (wood meal or effluent) is highly colored.
Another team of researchers, Bolton, et al. (“Mild Acid Pre-Treatment Ahead of SAQ Pulping of Birch Chips,” ESPRA, Syracuse, N.Y., Oct. 11, 2007 (herein “Bolton (2007C),” included by reference herein in its entirety), including the present inventor, reported on the mild acidolysis of sugar maple and birch chips ahead of SAQ pulping. Bolton (2007C) found that acid pretreatment and sodium carbonate neutralization followed by chemical pulping produced a more selective pulp, that is, a pulp having less undesirable lignin and more cellulose, than wood chips cooked without the acid and carbonate pretreatments.
According to the prior art, for example, as exemplified by the above patents of Elmore, Tikka, Sherman, and Prior, the hydroxide anion (OH−) to is used to neutralize acid pre-treated biomass, for example, wood chips. In contrast to this and related prior art, aspects of the present invention employ the carbonate anion (CO32−) for neutralization, which the present inventor shows provides superior pulp properties compared to pulp produced with hydroxide neutralization.
According to aspects of the present invention, the inventor has found that re-using the effluents from one or more of the acid pre treatment and the carbonate pretreatment provides further enhancements on the quality of the pulp produced by chemical pulping processes.